I still have my old 12 valve Cummins for a reason. Every time I get a newer model I lose fuel economy and the new trucks are higher maintenance. The 24 valve gets 14 mpg when my old 12 valve gets 19 mpg and has more power from mods. The Duramax is thirsty IMO, had three in the family, all had injector issues. The new Dodges also have the injector issues every 100K or so. Haven't looked at Fords in years, the 6.0 back in 2003 soured me on Ford diesels.
I would suggest a good used stock diesel truck. Do research and get the best of the bunch for the brand.
The reason I drive diesel trucks is the POWER. I have two 3500 duallys and a gas powered version couldn't touch either. With my 3500 4x4 I can pull about 12K and still get 11 mpg or better. I could get 14 mpg pulling a 31' travel trailer, or 9 mpg pulling my 18K pound 40' 5th wheel. It varies, pulling my Kubota
L4330 with a 6' cutter on the 14t flat deck is a 11 to 12 mpg happening if in town. Stop and go traffic kills the fuel level...
I always liked the Chevy with the large gas motor though, too bad they killed it off. That big block was a good bargain for RV pulling. The 9 mpg sucked though! I heavily lean towards a 305/555 vintage Dodge with the 6 speed, 4x4, 4.10 LS dually. Love our 2003.